S1PR5 and secondary progressive multiple sclerosis: A-971432 is a specific agonist of S1P5 provided by AbbVie, and siponimod (BAF312) from Novartis targets S1P1 and S1P5 and was approved in the USA for the treatment of secondary progressive multiple sclerosis [60,70], while other agents with S1P5 engagement may also be relevant (i.e., ozanimod, etrasimod) [13].